This invention relates to the brightening of wood pulps and, more particularly, relates to the brightening of sulphonate pulps.
The presence of various metallic substances in wood pulp is very common. The metallic substances can be from those originally present in the wood or contamination from process equipment, process water and chemicals artificially introduced. Wood pulp can be considered as a natural chelating agent, complexing metal ions mainly through lignin and other aromatic compounds. Numerous metal ions are known to form colored complexes, thereby lowering the pulp brightness and producing undesirable discolorations. Some metals present in pulp catalyse the decomposition of bleaching agents such as hydrosulphite and hydrogen peroxide. The presence of heavy metal ions causes brightness instability of paper.
In order to reduce the problems associated with metal contamination, application of chelating agents has been a common practice. Generally, the complexes formed are water soluble and should be removed from the system.
Japanese Kokai No. 12,822/80 assigned to G. Okuro described a treatment of kraft pulp with a metal salt including tin to avoid the emission of hydrogen sulphides, thereby inhibiting corrosion of silver, copper or lead wrapped in the paper. Stannous chloride, used together with disodium or sodium phosphate, is described in U.S.S.R. Pat. No. 787,518 to stabilize hydrogen peroxide in a pulping process. A combination of tin or other water soluble metallic compounds, lignosulphonate and cationic polymeric flocculant is described to be effective for biological oxygen demand (BOD) and chemical oxygen demand (COD) reduction of pulping effluent in Japanese Kokai No. 55701/77. Sodium stannate and other reducing agents in soda liquor have been used to cook black spruce chips to obtain a brighter pulp.
In common with other lignin-rich wood pulps, jack pine and spruce balsam ultra-high-yield (UHY) pulps prepared by digesting chips with sulphite under various conditions, followed by disc refining, are readily discolored by many metal ions commonly found in pulp and paper mills.